Need support on fast formula to fetch historical position details

Summary:

In Manage position page, we have defined Descriptive flex field (Bonus/Incentives) to Identify whether the position is Bonus eligible or Incentive eligible.

Based on the employee's position (in Assignment) this eligibility will be determined and values & eligibility will be processed in compensation accordingly.

However we have a case, where during the compensation evaluation period we may have position change for the particular employee (Old Position may be eligible for bonus, but new position is eligible for incentive).

In that case, employee will be eligible for both bonus & incentive in same evaluation period and values (for bonus & incentive) needs to be processed with Proration.